This invention relates to a circuit for processing an analog signal before analog to digital conversion, and more particularly to a biasing circuit of an amplifier.
An analog signal must be amplified, before analog to digital conversion, to a level appropriate for a reference voltage of the conversion. And a peak or a bottom of the analog signal is often clamped at a certain voltage level by a clamper. For example, when a television signal as shown in FIG. 3 is to be digitized, bottoms of horizontal synchronizing pulses must be clamped at a certain voltage level, and positive peaks of the amplified signal must be lower than Vcc, where Vcc is a voltage at a positive terminal of power supply to the amplifier, and negative peaks of the signal must be higher than GND(ground), where GND is a voltage at a negative terminal of the power supply to the amplifier.
A nonlinear A/D(analog to digital converter) was proposed on a Provisional Publication No. 87237/82 of a Japanese patent application entitled "A nonlinear A/D". FIG. 4 shows a block diagram of the nonlinear A/D, wherein a reference voltage Vrf for analog to digital conversion is not a constant voltage, but a voltage synthesized from a constant bias voltage Vbias and a mean value of an input signal Vin(mean), and can be expressed by Vrf=.alpha. Vin(mean)+Vbias. Thus, an accommodation between mean value of the signal voltage and the reference voltage is automatically obtained, and a variety of nonlinear characteristic is obtained by varying the coefficient .alpha..
But, clipping of peaks and bottoms can not be avoided by the nonlinear A/D. Referring to FIG. 4, an analog signal is terminated by a termination resister 1, and DC component of the analog signal is cut by a capacitor 2. The other electrode of the capacitor 2 is grounded through an input resistor(not shown in the drawing) of an amplifier 4. A biasing voltage is automatically generated by a mean value of current flowing in the input resistor. This automatic biasing can not prevent the clipping.
A low pass filter 8 produces a mean value Vin(mean) of the input signal, and this mean value is synthesized at a voltage synthesizer 9 to produce the reference voltage Vrf.
As shown by FIG. 5, when a mean value of the signal is low, a low biasing voltage is generated, and if a bottom of the signal comes at the low biasing, the bottom of the signal may be clipped, and when a mean value of the signal is high, a high biasing voltage is generated, and if a peak of the signal comes at the high biasing, the peak of the signal may be clipped. When the clipped bottom of the signal is a horizontal synchronizing pulse, the pulse is lost, and when the clipped peak of the signal is in a video signal, the waveform of the video signal is distorted.